(1) Field of the Invention
This invention generally relates to an apparatus for testing integrated circuits and more specifically to an apparatus for controlling the temperature of an integrated circuit under test.
(2) Description of the Prior Art
It is well known that the performance of an integrated circuit varies with the temperature of that integrated circuit. Consequently, integrated circuit specifications typically define a range of operating temperatures, such as from -55.degree. C. to +125.degree. C. As the temperature of an integrated circuit deviates from a nominal, normally room temperature, significant performance variations may occur if an integrated circuit is faulty. In some cases, the integrated circuit may even stop working when it is in a particular temperature range and thereafter may or may not return to its normal operation when the temperature returns to its nominal range.
As integrated circuits have become more complex and costly, there has been a heightened interest in testing them under widely varying temperature conditions. Such testing helps in acquiring an understanding of temperature effects both on the performance of the integrated circuit individually and on the performance of electronic assemblies incorporating a particular integrated circuit together with other integrated circuits. Moreover, such testing, as part of a quality control program, can improve electronic circuit assembly and manufacturing processes.
In recent years manufacturers have adopted rigorous quality control procedures during the initial manufacture of an integrated circuit on a wafer and at all subsequent intermediate assembly steps whereby that integrated circuit is included in a completed electronic assembly. Thermal testing at a wafer stage involves testing individual circuits or "dies" to determine whether the dies perform properly at specified high and low temperature limits.
U.S. Pat. No. 4,791,364 issued Dec. 13, 1988 to Kufis et al for a Thermal Fixture for Testing Integrated Circuits discloses such apparatus. Specifically, an indexing table positions a wafer with a number of individual dies in proximity to a testing probe. The probe includes a structure for directing, in a localized manner, a high-temperature, low-temperature or ambient-temperature gas, such as nitrogen, to an individual die. Gas can be used for temperature adjustment because it is only necessary to heat or cool the wafer surface in order to change the temperature of a die. The apparatus initially directs cold gas to a die under test and monitors performance after the temperature stabilizes. Next the apparatus directs hot gas to the die and monitors performance after the temperature stabilizes. Testing stops if either test indicates improper performance. If a die passes both the high and low temperature tests, gas at an ambient temperature is directed to the area merely for purposes of cooling the die.
Integrated circuit manufacturers combine various dies into integrated circuits encased in a ceramic or other housing with a plurality of terminal pins for assembly with other integrated circuits on a printed circuit board. Testing continues during the assembly process. However, apparatus such as disclosed in the Kufis et al patent is not effective because the housings on integrated circuits tend to insulate the dies. Quality control procedures involving thermal testing also become more complex when assembled printed circuit boards are under test. Particularly, a circuit board under extreme thermal conditions can fail intermittently and produce seemingly unrelated changes in system performance thereby complicating circuit diagnoses and repair. In one prior art approach an entire circuit board to be is located in an environmental chamber. The chamber controls the temperature of the circuit board and all the components. It is not possible to alter the temperature of a single integrated circuit independently in such a chamber. Yet it can be difficult to determine which individual integrated circuit produces erratic results without the capability of altering the temperature on a single integrated circuit. In some situations circuit timing signals may prevent testing of even a single circuit board in an environmental chamber remotely from other printed circuit boards.
Heat guns and cooling liquids, such as Freon, are also used to direct temperature controlling substances toward an individual integrated circuit. Although this localizes the temperature change to an area surrounding a particular integrated circuit, it is difficult to stabilize the temperature of an individual integrated circuit for any significant interval. The results therefore tend to be qualitative rather than quantitative. Cooling liquids also can produce condensation on the printed circuit board and integrated circuit. Such condensation can cause circuits on a printed circuit board to malfunction.
Other apparatus uses a cooling unit that connects directly clamps to an integrated circuit using a mechanical clamping mechanism. No analogous structures have been suggested for heating. These cooling units are cumbersome to use, and the time required to clamp and unclamp an individual cooling unit from an integrated circuit can dramatically extend the time required to test a circuit board.